Often sensors are used in applications, where ultimate reliability and prevention of misuse or fraudulent manipulation is crucial: life sustaining applications in medical treatment, applications in transportation where lives may be endangered in case of malfunction, metering, billing and remote payment systems which need protection against forgery or fraudulent falsification.
Applications for magnetic sensors in general and for differential magnetic sensors in particular are, for example, systems counting the rotations of a mechanical member, for example for measuring the amount of water flowing through a pipe or measuring the number of turns of a wheel in a car, that must be protected against manipulation by electromagnetic stimuli. For example, one could try to apply a rotating magnetic field to such a sensor system to imitate the rotation of the mechanical member thereby manipulating the number of rotations detected. The rotating field could be generated by attaching a permanent magnet to a handheld drilling machine or by using two orthogonal coils supplied with two sinusoidal currents with 90° phase shift.
Another example refers to electricity meters, where one could try to attach a small permanent magnet nearby a sensor in an intend to defraud. If the current through a conductor is measured, for example using a magnetic sensor, one could try to bend the conductor so that the current flows in opposite direction and close to the original sensor thereby reducing the magnetic field on the sensor which would decrease the measured value of apparent current.
Besides intentional misuse as described above, it is beneficial if these sensor systems are also robust against unintended manipulation, or put more general, against abnormal operating conditions. For example, if a rotational position sensor is exposed to a large magnetic field this may impair its accuracy. In an automotive system this may lead to a wrong ignition timing with increased fuel consumption and increase air pollution. In medical instrumentation this may lead to inaccurate determination of a three dimensional location (3D-location) of a micro-surgery tool during a delicate heart- or brain-surgery.
Therefore, there is a need to make sensors or systems using such sensors robust against manipulation or against an abnormal operating condition and/or to detect, whether a manipulation or an abnormal operating condition occurs.